Droplet-on-demand ink jet printing systems eject ink droplets from print head nozzles in response to pressure pulses generated within the print head by either piezoelectric devices or thermal transducers, such as resistors. The ejected ink droplets, commonly referred to as pixels, are propelled to specific locations on a recording medium where each ink droplet forms a spot on the recording medium. The print heads have droplet ejecting nozzles and a plurality of ink containing channels, usually one channel for each nozzle, which interconnect an ink reservoir in the print head with the nozzles.
In a typical piezoelectric ink jet printing system, the pressure pulses that eject liquid ink droplets are produced by applying an electric pulse to the piezoelectric devices, one of which is typically located within each one of the ink channels. Each piezoelectric device is individually addressable to enable a firing signal to be generated and delivered for each piezoelectric device. The firing signal causes the piezoelectric device receiving the signal to bend or deform and pressurize a volume of liquid ink adjacent the piezoelectric device. As a voltage pulse is applied to a selected piezoelectric device, a quantity of ink is displaced from the ink channel and a droplet of ink is mechanically ejected from the nozzle associated with each piezoelectric device. The ejected droplets are propelled to pixel targets on a recording medium to form an image on an image receiving member opposite the print head. The respective channels from which the ink droplets were ejected are refilled by capillary action from an ink supply.
In some printers, the image receiving member is a rotating drum or belt coated with a release agent. The print head ejects droplets of melted ink onto the rotating image receiving member to form an image, which is then transferred to a recording medium, such as paper. The transfer is generally conducted in a nip formed by the rotating image member and a rotating pressure roll, which is also called a transfix roll. The pressure roll may be heated or the recording medium may be pre-heated prior to entry in the transfixing nip. As a sheet of paper is transported through the nip, the fully formed image is transferred from the image receiving member to the sheet of paper and concurrently fixed thereon. This technique of using heat and pressure at a nip to transfer and fix an image to a recording medium passing through the nip is typically known as “transfixing,” a well known term in the art.
Ink jet printers are capable of producing either simplex or duplex prints. Simplex printing refers to producing an image on only one side of a recording medium. Duplex printing produces an image on each side of a recording medium. In duplex printing, the recording medium passes through the nip for the transfer of a first image onto one side of the recording medium. The medium is then routed on a path that presents the other side of the recording medium to the nip. By passing through the nip again, an image is transferred to the other side of the medium. When the recording medium passes through the nip the second time, the side on which the first image was transferred is adjacent to the transfix roll. Release agent that was transferred from the image receiving member to the recording medium may now be transferred from the first side of the recording medium that received an image to the transfix roll. Thus, a duplex print transfers release agent to the transfix roll and multiple duplex prints may cause release agent to accumulate on the transfix roll.
The amount of release agent on the transfix roll may reach a level that enables release agent to be absorbed by the back side of a recording medium while an image is being transfixed to the front side of the recording medium. If a duplex print is being made, the side of the recording medium receiving a second image may now have release agent on it. The release agent on the recording medium may interfere with the efficient transfer of ink from the image receiving member to the recording medium. Consequently, ink may remain on the image receiving member rather than being transferred to the recording medium. This inefficient transfer of ink may produce an image in which partial or missing pixels are noticeable. This phenomenon is known as image dropout. Additionally, ink remaining on the image receiving member may require the image receiving member to undergo a cleaning cycle. Otherwise, the ink not transferred from the image receiving member may interfere with the formation of a subsequent image on the image receiving member.
To aid in the transfer of ink from the image receiving member to the second side of a recording medium, some printers transfix all duplex images at a rotational speed that is slower than a rotational speed used for simplex printing. The slower speed exposes the medium in the nip to the pressure in the transfer nip longer and that exposure helps improve the efficiency of the image transfer to recording media having release agent on the surface of the media. The slower speed of the duplex printing process, however, reduces printer throughput during duplex printing operations. Therefore, performing duplex printing in a manner that improves throughput without subjecting image quality to dropout and the like is useful.
Application of release agent to the imaging member also affects image quality. When the applicator that applies release agent to the imaging member contacts the imaging member while it rotates at higher rotational speeds, more release agent is deposited on the imaging member. Slower rotational speeds result in less release agent being applied to the imaging member. Consequently, the imaging member speed also affects the amount of release agent available for absorption by the front side of a media sheet during a duplex printing cycle.
Excessive use of release agent not only contributes to image dropout, but may also shorten the life of a consumable module known as the cleaning unit. The process of applying release agent to the image receiving member, in terms of rotational speed and timing, also affects how much release agent is consumed during printing. Therefore, regulating the application of release agent to an image receiving member also contributes to conservation of release agent and extension of the operational life of the cleaning unit.